Plasmons are understood to be a quantum of plasma oscillation. Plasmons may be classically described as an oscillation of a free electron density against a fixed positive ion in a metal. Electrons may move in one direction (uncovering positive ions) in the presence of an external electric field until they cancel the field inside the metal. In the absence of the electric field the electrons may shift and be repelled by one another and attracted to the positive ions. The electrons may then oscillate back and forth at the plasma frequency until energy may be lost in some kind of resistance or damping. Plasmons are the quantization of this type of oscillation.
Surface plasmons, which may also be known as surface plasmon polaritons (SPPs), are surface electromagnetic waves that propagate in a direction parallel to a metal/dielectric or metal/vacuum interface. As the wave is on the boundary of the metal and external medium (air or water for example), these oscillations may be very sensitive to any change of this boundary, such as the adsorption of molecules to the metal interface.